Method and apparatus for the production of particulate carbon products

ABSTRACT

This invention relates to a method for the production of particulate carbon products in a reactor vessel wherein gas flow between a gas inlet port and a gas outlet port suspends a bed of catalyst-containing particulate material in said vessel and wherein the product is discharged from said vessel by falling from the bed.

This application is a continuation application of pending U.S.application Ser. No. 10/580,231, filed Feb. 9, 2007 (of which the entiredisclosure of the pending, prior application is hereby incorporated byreference).

The present invention relates to a method and reactor and in particulara method and reactor suitable for continuous production of products suchas carbon nano-fibres (CNF) and hydrogen.

It has long been known that the interaction of hydrocarbon gas and metalsurfaces can give rise to dehydrogenation and the growth of carbon“whiskers” on the metal surface. More recently it has been found thatsuch carbon whiskers, which are hollow carbon fibres having a diameterof about 3 to 100 nm and a length of about 0.1 to 1000 μm, haveinteresting and potentially useful properties, e.g. the ability to actas reservoirs for hydrogen storage (see for example Chambers et al. inJ. Phys. Chem. B 102: 4253-4256 (1998) and Fan et al. in Carbon 37:1649-1652 (1999)).

Several researchers have thus sought to produce carbon nano-fibres andto investigate their structure, properties and potential uses and suchwork is described in a review article by De Jong et al in Catal.Rev.—Sci. Eng. 42: 481-510 (2000) which points out that the cost of theCNF is still relatively high (ca. US $50/kg or more). There is thus aneed for a process by which CNF may be produced more efficiently.

As described by De Jong et al. (supra) and in a further review articleby Rodriguez in J. Mater. Res. 8: 3233-3250 (1993), transition metalssuch as iron, cobalt, nickel, chromium, vanadium and molybdenum, andtheir alloys, catalyse the production of CNF from gases such as methane,carbon monoxide, synthesis gas (ie H₂/CO), ethyne and ethene. In thisreaction, such metals may take the form of flat surfaces, ofmicro-particles (having typical sizes of about 100 nm) or ofnano-particles (typically 1-20 nm in size) supported on an inert carriermaterial, e.g. silica, alumina, titania, zirconia or carbon. The metalof the catalyst must be one which can dissolve carbon or form a carbide.

Both De Jong et al (supra) and Rodriguez (supra) explain that carbonabsorption and CNF growth is favoured at particular crystallographicsurfaces of the catalyst metal.

Although methods of producing small amounts of carbon products such ascarbon nano-fibres are known in the art, methods of producing largequantities efficiently and with reliable quality have so far proveddifficult to realise, particularly on an industrial scale.

Existing techniques for the synthesis of products such as carbonnano-fibres (CNF) include arc discharge, laser ablation and chemicalvapour deposition. These techniques generally involve vaporising carbonelectrodes at elevated temperatures. For example, the laser ablationtechnique involves using a laser to vaporise a graphite target in anoven. The arc discharge technique involves carbon rods, placed end toend, which are vaporised in an inert gas.

Many of these techniques involve batch processes which do not producereliable and consistent carbon product quality in any great volume. Forexample, arc discharge production methods often produce CNF productswhich have a random size distribution and therefore require substantialpurification. Laser ablation techniques on the other hand require highpower sources and expensive laser equipment which leads to a high unitcost of product delivered by this technique.

Fluidised bed reactors have been considered as a means to alleviate someof these problems associated with synthesising carbon and particulateproducts. However, the large scale production of carbon products, and inparticular CNF products with uniform product size and quality, hasproved difficult to achieve using conventional reactors. Fluidised bedreactors suffer from the difficulties of harvesting the synthesisedproduct from the fluidised region and in particular do not allowproducts of a certain size to be harvested efficiently from the reactionregion. Typically the harvested products will comprise a mixture ofproduct quality, some having had a longer reaction time in the bed thanothers. This does not provide a reliable output product from thereactors.

There is therefore a need for a method and a reactor, capable ofoperating continuously, which can efficiently and reliably produceparticulate carbon products.

Thus, viewed from a first aspect, the present invention provides amethod for producing a particulate carbon product in a reactor vesselwherein gas flow between a gas inlet port and a gas outlet port suspendsa bed of catalyst-containing particulate material in said reactor vesseland said particulate carbon product is discharged from the reactorvessel by falling from the bed, e.g. through a particulate productoutlet port arranged beneath the bed.

Viewed from a second aspect, the present invention provides a reactorcomprising a vessel having a gas inlet port, a gas outlet port and aparticulate product outlet port, said gas inlet port being arranged suchthat in use gas flow therefrom suspends a bed of catalyst containingparticulate material in said vessel and particulate product isdischarged from the reactor by falling from the bed, e.g. through theparticulate product outlet port.

In effect the reactor can be seen to be an ‘inverted’ fixed or fluidisedbed reactor since, unlike conventional fixed or fluidised bed reactors,the reaction bed or region is formed in the reactor vessel without amechanical support so that the particulate product can be harvested onceit falls from the reaction bed.

The reaction bed may be a fluidised bed or alternatively may be a fixedbed, or simply a region of flowing gas in which the particles areentrained in the gas. The nature of the reaction bed depends on the gasflow rate and on whether the gas flows through a barrier which is gaspermeable but essentially impermeable to the particles. Where such abarrier is present, at sufficiently high gas flow rates a fixed reactionbed will be formed underneath the barrier.

The reactor may be provided with means to prevent the particulateproduct and/or catalyst from leaving the reactor through the gas outletport. Preferably, the reactor is provided with means to allow the outletgas to leave the reactor but to retain the product and/or catalystwithin the reactor. This may thus function as the barrier mentionedabove.

Alternatively where the product and/or catalyst leaves the reactorvessel through the gas outlet port the reactor may be provided withmeans to return the product and/or catalyst to the reactor vessel. Forexample, the reactor may be provided with a cyclone or radiclone intowhich the outlet gas is fed and which removes the particulate productand/or catalyst from the outlet gas flow. The reactor may then beprovided with means to return the product and/or catalyst to thereaction bed.

Preferably the reactor is provided with a filter or gas permeablebarrier through which the outlet gas flows to retain the product and/orcatalyst upstream of the filter or barrier.

The gas permeable barrier may be arranged in the gas outlet pipe orconduit of the reactor or alternatively within the reactor vesselitself. When located in the reactor vessel the gas permeable barrier maybe located between the gas outlet and gas inlet such that the reactionregion is formed below the lower surface of the gas permeable barrier.

The gas permeable barrier is preferably located towards the top of thereactor vessel and more preferably defines the top of the reactorvessel. In this arrangement the gas permeable barrier can extend acrossthe entire cross-section of the reactor vessel thereby maximising thefiltering area and reducing the gas velocity through the barrier and thepressure drop across the barrier.

The catalyst and particulate product are supported and suspended in thereactor vessel and in the reaction region by the flow of gas through thereactor vessel. The flow rate of gas may therefore be controlled so asto vary the size of the product discharged from the reaction region andfrom the reactor vessel.

The gas flow rate is preferably selected so that a region is providedbetween the reaction region and the gas permeable barrier where littleor no particulate material is present, i.e. a region where little or noreaction occurs. A gas-suspended fluidised bed or reaction region cantherefore be generated in this way. Alternatively a higher gas flow ratemay be selected so that the reaction bed or region is located againstthe gas permeable barrier. An inverted fixed reaction bed or region canthereby be formed.

The permeability of the barrier, i.e. the pore size, aperture size orminimum diameter of the gas flow path through the barrier, is preferablyselected to prevent the particulate material in the reaction regionpassing through the barrier. Especially preferably it is selected toprevent catalyst-containing particles that are fed into the reactorbefore or during operation from passing through the barrier.

While the barrier may be perforated metal, it is preferably a porousceramic. Alternatively, the barrier may be a filter formed from carbonnano-fibres or glass fibres.

The reactor vessel may also be provided with means to provide a backpressure to reverse the flow of gas through the gas permeable barrier orfilter in order to unblock any blocked pores or apertures. Typicallythis may be achieved by providing the top of the reactor with a gasinlet port through which pressurised gas can be introduced into thevessel and which can flow through the gas permeable barrier in a reversedirection, i.e. gas flow in an opposite direction to gas flow when thereactor is in normal operation. A back pressure may be provided duringoperation of the reactor by pulsing a reverse gas flow or alternativelyby stopping the reaction and providing a reverse gas flow.

It will be appreciated that the reactor vessel may be provided with morethan one gas inlet port and with more than one gas outlet port.

To minimise catalyst deactivation, the inlet gas (or feed gas) ispreferably fed into the reactor vessel and the reaction region at aplurality of points around the reaction region. The reaction region gasinlet port(s) may be arranged tangentially to the inner surface of thevessel so as to introduce gas into the reaction bed at an angle and tospin or rotate the reaction bed.

Alternatively, the reaction region gas inlet ports may be arranged atvarying angles to the inner surface of the vessel so as to agitate thereaction region. These inlet ports moreover may be disposed away fromthe reactor vessel inner walls towards or at the vessel centre. In thisway gas may be introduced within the reaction bed itself. If thisarrangement is adopted, the gas conduits extending into the reactorvessel are preferably made of or coated with a ceramic material toreduce surface corrosion.

The particulate catalyst may be introduced into the vessel via the gasinlet port. Alternatively, the vessel may be provided with one or morecatalyst inlet ports through which the catalyst can be introduced.

Preferably, a catalyst inlet port introduces catalyst into the vesselproximate the reaction region so that the catalyst is dispersed into thereaction region. Alternatively the catalyst may be introduced into alower temperature and or pressure region within the reactor vessel. Thecatalyst may be introduced into the reactor in a powder form using a gasor alternatively may be introduced into the reactor as or using aliquid.

The catalyst may be introduced continuously or batch-wise.

The catalyst may be introduced into the reactor vessel entrained in acarbonaceous feed gas; however to reduce carbon deposition in the feedlines, it will generally be preferred to use a gas or liquid carrierwhich does not react with the catalyst. Nitrogen may thus be used as acarrier in this regard.

The vessel may be provided with more than one product outlet portalthough in general it is believed one will be sufficient.

The vessel may have a product collection area arranged at the bottom ofthe reactor vessel and may also have means to remove product from thereactor or product collection area.

Particularly preferably the product outlet port leads to a particulateproduct collection vessel which is isolatable from the reactor vessel,e.g. to permit removal of the collection vessel from the reactor or topermit removal of the product from the collection vessel (e.g through aproduct removal port in the collection vessel). The collection vesselwill preferably be provided with a cooling means, e.g. a cooling jacket.Especially preferably the cooling means is a heat exchanger whereby heatmay be transferred from the product to the feed gas.

The reactor may be arranged at any angle where the particulate productoutlet port is located beneath the reaction region such that theparticulate product is discharged from the reactor vessel by fallinginto a collection area from the reaction region. Preferably the reactoris arranged so that the particulate product outlet port is arrangedvertically beneath the reaction region.

The reactor vessel may be surrounded by an outer casing surrounding andsupporting the vessel. The outer casing, gas inlet, gas outlet and theparticulate product outlet port (and associated conduits) may bemanufactured from a high temperature steel.

The gas inlet and outlet ports and the particulate product outlet port(and associated conduits) are preferably manufactured from a steel witha silicon content of between 1.8% and 2.3% and a chromium content ofgreater than 30%. Sophisticated materials with more than 2.5% aluminium,e.g APM, APMt (manufactured by Sandviks) or MA956 (manufactured bySpecial Metals) may also be used. Conventional chromium based tubing canbe used to reduce the iron fraction of the metal surface and therebyreduce the tendency towards dusting or carbon deposition on the surfaceof the tubing or conduits. The reactor vessel may also be manufacturedfrom similar material. Preferably however the reactor vessel ismanufactured from or lined with a high temperature resistant castableceramic material such as, for example, Ceramite

manufactured by Elkem ASA, Norway.

The reaction within the reactor vessel may take place at ambienttemperature and pressure. Preferably however the reactor operates at anelevated temperature and pressure. Preferably the reactor operatesbetween 2 and 25 bar and more preferably between 5 and 20 bar. Mostpreferably the reactor operates between 5 and 15 bar. The reactor maytypically operate at a temperature of up to 1000

C. Preferably the reactor operates in the range 400

C to 900

C and most preferably in the range 550

C to 900

C. In this context, temperature and pressure refer to temperature andpressure in the reaction bed.

The outer casing may be internally pressurised to a pressure equal tothe pressure within the reactor vessel. This is particularlyadvantageous where a ceramic vessel is used. Pressure equalising theinner and outer reactor vessel walls reduces stresses within the ceramicmaterial when reaction in the reactor takes place at elevated pressures.The outer casing may further be provided with an insulating layerbetween the outer casing and the reactor vessel outer wall. Theinsulating material may, for example, be an insulating mineral wool orsome other suitable insulating material.

Where endothermic reactions take place within the reactor vessel thereactor may be provided with means to heat the reaction region and/orgas within the reactor vessel. The heating means may be heating coilsfor example and may be integrated into the wall of the reactor vessel.The heating means may, for example, be arranged in cavities or apertureswithin a ceramic reactor vessel.

Alternatively, heating coils may be arranged around the exterior of thevessel or within the reactor vessel itself.

Where the reaction is endothermic, heat is preferably also provided intothe reaction region by introducing the feed gas into the reactor vesselat elevated temperature. It is especially preferred in this respect tointroduce the feed gas within the reaction region as well as before thereaction region as in this way the required feed gas inlet temperaturemay be reduced so reducing the risk of catalyst deactivation. Where oneof the gases making up the feed gas is reactive with ferrous metals atelevated temperatures, e.g. where carbon monoxide is used, it willgenerally be desirable to introduce such a gas at a lower temperaturethan that used for the remaining gases.

As mentioned above, the reactor may further include means to cool theparticulate product leaving the reactor vessel. For example, the reactormay be provided with a cooling cavity or jacket surrounding theparticulate product outlet port of the reactor or arranged adjacent tothe product outlet port. The cooling cavity may be provided with acontinuous flow of coolant such as water or feed gas which reduces thetemperature of the product leaving the reactor vessel. Other coolantscan equally be employed in the cooling cavity to cool the product.

A reactor according to the present invention may be used particularlyadvantageously in the production of carbon products and in particularcarbon products such as carbon nano-fibres (CNF).

Thus, viewed from another aspect, the invention provides a reactorarranged to produce carbon nano-fibres comprising a vessel having a gasinlet port, a gas outlet port and a particulate carbon product outletport, said gas inlet port being arranged such that in use gas flowtherefrom suspends a bed of catalyst-containing particulate material insaid vessel and particulate carbon product is discharged from the vesselby falling from the bed, e.g. through the particulate product outletport.

The reactor may conveniently have a volume of 10 to 100 m³, preferably50 to 70 m³ allowing a total product content in the thousands ofkilograms. For continuous operation, inlet gas feed rates of 500 to 2000kg/hour, eg 1000 to 1500 kg/hour, and product removal rates of 200 to2000 kg/hour, eg 750 to 1250 kg/hour may thus typically be achieved. Theenergy supply necessary to operate such a reactor for the production ofcarbon will typically be in the hundreds of kW, eg 100 to 1000 kW, moretypically 500 to 750 kW. Alternatively expressed, the energy demand willtypically be in the range 1 to 5 kW/kgC.hour⁻¹, e.g. 2-3.5kW/kgC.hour⁻¹.

Any suitable catalyst may be used in the production of CNF which candissolve carbon or form a carbide and which is capable of beingsuspended in the gas flow within the reactor.

The catalyst may be any transition metal such as iron, cobalt, nickel,chromium, vanadium and molybdenum or other alloy thereof. Preferably thecatalyst is an FeNi catalyst. The catalyst may be supported on an inertcarrier material such as silica, alumina, titania, zirconia or carbon.

More preferably the catalyst used is a porous metal catalyst comprisinga transition metal or an alloy thereof, e.g. as described inPCT/GB03/002221, a copy of which is filed herewith and the contents ofwhich are hereby incorporated by reference. The use of the Raney metalcatalysts described in PCT/GB03/002221 especially the Amperkat

catalyst mentioned therein is especially preferred.

In order that the catalyst particles fulfil certain aerodynamic criteriathe catalyst may be pre-treated prior to entering the reactor vessel inorder to increase the drag on the catalyst.

The catalyst may also be pre-treated to increase carbon production rateand carbon yield and this may be achieved with any carbon productioncatalyst, i.e. not just porous metal catalysts, by a limited period ofexposure to a feed gas with reduced or no hydrogen content at a lowertemperature than the reaction temperature in the main carbon productionstage. Such pre-treatment is preferably under process (i.e. reactor)conditions under which the carbon activity of the catalyst is greaterthan in the main carbon production stage. This process thus comprises ina first stage contacting a catalyst for carbon production with a firsthydrocarbon-containing gas at a first temperature for a first timeperiod and subsequently contacting said catalyst with a secondhydrocarbon-containing gas at a second temperature for a second timeperiod, characterised in that said first gas has a lower hydrogen (H₂)mole percentage than said second gas, said first temperature is lowerthan said second temperature, and said first period is shorter than saidsecond period. If a higher graphitic contact of the carbon product isdesired, the first temperature may be reduced and/or the secondtemperature may be increased.

The temperature in the first period is preferably in the range 400 to600

C, especially 450 to 550

C, more especially 460 to 500

C. The hydrogen mole percentage in the first period is preferably 0 to2% mole, especially 0 to 1% mole, more especially 0 to 0.25% mole,particularly 0 to 0.05% mole. The pressure in the first period ispreferably 5 to 15 bar, especially 6 to 9 bar. The duration of the firstperiod is preferably 1 to 60 minutes, more especially 2 to 40 minutes,particularly 5 to 15 minutes. The temperature, pressure and gascomposition, in the second period are preferably as described above forthe reactor.

Pre-treatment or initiation of the catalyst causes the catalyst tobecome a catalyst/carbon agglomerate comprising particles of acarbon-containing metal having carbon on the surfaces thereof. Beforethis pre-treatment, the catalyst may if desired be treated with hydrogenat elevated temperature, e.g. to reduce any surface oxide.

The gas flowing from the gas inlet to the gas outlet may be any suitablegas for sustaining the reaction in the reaction region. For CNFproduction the gas may be any C₁₋₃ hydrocarbon such as methane, ethene,ethane, propane, propene, ethyne, carbon monoxide or natural gas or anymixture thereof. Alternatively, the gas may be an aromatic hydrocarbonor napthene.

The inlet gas may also include a proportion of hydrogen to reduce thecarbon activity of the catalyst metal, i.e. the rate of carbon uptake bythe metal. The gas may typically contain 1 to 20% mole of hydrogen.Preferably the gas contains 2 to 10% mole hydrogen.

The inlet gas may include carbon monoxide. However, carbon monoxide ispreferably introduced at a lower temperature (e.g. <300

C), for example through a separate feed line, e.g. to avoid dusting offerrous metal feed lines which can occur at temperatures above 400

C. Carbon monoxide is a desirable component of the feed gas as thereaction to produce carbon is less endothermic than that of methane forexample.

When carbon monoxide is introduced into the reactor vessel through aseparate gas inlet, the main feed gas inlet may have a correspondinglyhigher inlet temperature such that the gases mix in the reactor vesselto produce a mixture at the appropriate temperature.

Where the feed gas passes through metal pipes or conduits (such as ironor chromium based metals or alloys), the oxide layer on the surface ofthe pipe or conduit (which acts to protect the metal) can be maintainedby introducing a small quantity of an oxygenaceous compound (e.g. wateror CO₂) into the feed gas.

The inlet or feed gas may be recirculated completely or partially fromthe gas outlet back to the gas inlet. Alternatively the gas may flowthrough the reactor once. More preferably a proportion of gas isrecirculated internally within the vessel. Internal recirculation (orbackmixing) of the gas within the reactor can be used to control thehydrogen content within the reactor and thus reduce the amount ofhydrogen which needs to be introduced into the reactor vessel.

Gas removed from the reactor vessel is preferably passed through aseparator in which hydrogen is removed by metallic hydride formation.Pellets of a metallic hydride in a column absorb the produced hydrogenat a low temperature, and the absorbed hydrogen can then be recovered byraising the temperature in the column.

Excess hydrogen may alternatively be removed by passing the gas past amembrane, polymer membrane or pressure swing absorber (PSA). Themembrane may for example be a palladium membrane. Hydrogen retrieved inthis way may be an end product of the carbon production reaction or itmay be burned to provide energy, e.g. to heat the feed gas.

On the small scale, energy supply into the reactor may be achieved byexternally heating the reactor vessel or by inclusion within the reactorof heating means or heat exchange elements connected to a heat source.The heating means may for example be electrically powered heating coilsand may be integrated into the wall of the reactor vessel. The heatingmeans may be arranged in cavities or apertures within the ceramicmaterial.

As reactor size increases however it will become more necessary to heatthe inlet or feed gas that is supplied to the reactor vessel.

The gas may be partially pre-heated or completely pre-heated to thereactor operating temperature before it enters the reactor vessel.Preferably the gas is part pre-heated before entering the reactor vesseland heated further to the operating temperature inside the reactorvessel using the reactor heating means. The gas may be pre-heated byheat exchange from the gas outlet flow leaving the reactor vessel.

The gas flowing from the gas outlet which is not recycled back into thereactor vessel may be incinerated or may, alternatively, be fed into ahydrocarbon gas stream to be used as a fuel gas or sales gas providedthat the level of hydrogen is acceptable.

The carbon produced in the reactor may be processed after removal fromthe reactor vessel, e.g. to remove catalyst material, to separate carbonfibres from amorphous material, to mix in additives, or by compaction.Catalyst removal typically may involve acid or base treatment; carbonfibre separation may for example involve dispersion in a liquid andsedimentation (e.g. centrifugation), possibly in combination with othersteps such as magnetic separation; additive treatment may for exampleinvolve deposition of a further catalytically active material on thecarbon, whereby the carbon will then act as a catalyst carrier, orabsorption of hydrogen into the carbon; and compaction may be used toproduce shaped carbon items, e.g. pellets, rods, etc.

Processing of the carbon product to reduce the catalyst content thereinmay also be achieved by heating, e.g. to a temperature above 1000

C, preferably above 2000

C, for example 2200 to 3000

C. The total ash content is also significantly reduced by thistreatment.

Catalyst removal from the carbon product may also be effected byexposure to a flow of carbon monoxide, preferably at elevatedtemperature and pressure, e.g. at least 50

C and at least 20 bar, preferably 50 to 200

C and 30 to 60 bar. The CO stream may be recycled after deposition ofany entrained metal carbonyls at an increased temperature, e.g. 230

to 400

C.

As a result of such temperature and/or carbon monoxide treatment anespecially low metal content carbon may be produced, e.g. a metalcontent of less than 0.2% wt, especially less than 0.1% wt, particularlyless than 0.05% wt, more particularly less than 0.01% wt, e.g. as low as0.001% wt.

The reactor vessel is preferably arranged in a vertical orientationcomprising a lower conical section, a middle cylindrical section and anupper inverted conical section such that the reduced cross-sectionalarea of the middle section increases the gas velocity and the increasedcross-sectional area of the upper section decreases the gas velocity;this acts to prevent particles leaving the upper section. This “waisted”arrangement is in itself novel and inventive.

Thus, viewed from yet another aspect an invention described hereinprovides a reactor comprising a vessel having a lower section having agas inlet port and defining a particulate product outlet port, an uppersection having a gas outlet port and defining a reaction bed and amiddle section connecting said upper and said lower sections wherein inuse gas flow from said lower section through said middle section to saidupper section suspends a bed of catalyst-containing particulate materialin said bed and particulate product is discharged from the vessel byfalling from the bed.

Preferably the middle section has a smaller cross-sectional area thanthe upper and lower sections. More preferably, the lower section has aconical shape, the middle section has a cylindrical shape and the uppersection has an inverted conical shape. Thus, in effect, the interior ofthe reactor has a ‘waisted’ or ‘hour glass’ shape. The conical sectionmay, in a preferred embodiment, be attached at both ends to cylindricalsections.

Thus, the flow rate of gas through the reactor can be used to regulatethe weight of the particles being discharged from the reactor.

The use of gravity to harvest products from a reactor can also beemployed in a reactor vessel containing a plurality of horizontallyarranged reaction beds in combination with a suitably disposedparticulate product outlet port.

Thus, a further invention disclosed herein provides a reactor comprisinga vessel having a gas inlet port and containing a gas outlet port and aplurality of reaction surfaces wherein in use a product is synthesisedon each of said reaction surfaces and is discharged from the vessel byfalling from the reaction surfaces.

The term reaction surface is intended to mean a surface, region or bedon or in which a reaction of a gas catalysed by a catalyst occurs.

The reactor may be provided with a single gas inlet port or, morepreferably, each of the reaction surfaces may be provided withindividual gas inlet ports so as to feed gas directly onto each of thereaction surfaces.

The reaction surfaces may be substantially horizontal and may beconfigured in a ‘tiered’ arrangement such that product falling from anupper surface falls onto a subsequent lower surface and eventually tothe bottom of the reactor.

The reaction surfaces may have increasing size towards the bottom of thereactor so that the product cascades from the upper reaction surfaces tothe lower reaction surfaces. Alternatively each of the reaction surfacesmay be the same size and may be provided with holes or apertures throughwhich the product can fall either onto the surface below or directly tothe bottom of the reactor by falling from the edge of a reactionsurface.

Catalyst may be introduced into the reactor as described with referenceto the reactors described above.

As discussed above, it is important to be able to add heat to thereaction region particularly where endothermic reactions take placewithin a reaction region or bed. It is therefore desirable to provide areactor with a number of gas inlets which can introduce heated feed gasinto a reaction region.

This can be achieved for reactors, other than those described above,wherein a reactor vessel is provided with a plurality of gas inlet portsor orifices.

Thus, a further invention disclosed herein provides a reactor comprisinga vessel having a plurality of gas inlet ports, a gas outlet port and aparticulate product outlet port, wherein in use a reaction bed is formedin said vessel containing a bed of catalyst-containing particulatematerial and said gas inlet ports are disposed so as to introduce gasinto the reaction bed.

The gas may be introduced directly into the reaction bed, for exampleusing a conduit extending into the region, or may alternatively beintroduced through ports in the vessel wall proximate the reaction bed.The gas may be introduced into the reaction region at any angle.

The reactor vessel may be arranged at any angle. Preferably the reactorvessel is arranged in a horizontal orientation; alternatively it may bearranged at an angle up to 45

from the horizontal.

The reactor vessel may be provided with gas inlet ports arranged suchthat in use gas flow therefrom suspends the bed of catalyst-containingparticulate material in said vessel and particulate product isdischarged from the reactor vessel by falling from the bed and throughone or more particulate product outlet ports.

The product outlet ports may be arranged along the base of the vessel inthe direction of travel of the bed such that particulate products can beharvested from the reactor by falling from the bed. Alternatively thegas outlet port and particulate product outlet port may be a commonoutlet port at the downstream end of the vessel.

The reactor may also preferably be provided with gas inlet and/or gasoutlet ports above and/or along the reaction bed.

The vessel may further be arranged so as to have an increasingcross-sectional area in the direction of gas flow. More preferably thevessel may be cylindrical or conical in shape.

With reference to the reactors discussed above, the gas inlet ports maybe arranged tangentially to the reactor vessel so as to agitate or spinthe reaction bed. For example the gas inlet ports may be arranged at 45

to the reactor vessel wall.

The reactor vessel may be static or may alternatively be arranged torotate so as to agitate the reaction bed. In such an arrangement, theinside of the reactor vessel may be provided with stirring members ormeans connected to the inside of the reactor vessel such that the bed isagitated and stirred as the vessel rotates. This arrangement can be usedto improve temperature distribution in the bed and/or to change theproduct size by erosion of the product.

Thus, gas can be provided along the length of the reaction regionthereby improving the efficiency of the reaction.

Preferred embodiments of the invention will now be described, by way ofexample only, and with reference to the accompanying drawings in which:

FIG. 1 shows a schematic of a reactor according to a first embodiment.

FIG. 2 shows a cut-away of the reactor vessel.

FIG. 3 shows a simplified diagram of the reactor and the three sectionsof the preferred embodiment of the reactor.

FIG. 4 shows a serial arrangement of reactors.

FIG. 5 shows a tiered reactor arrangement.

FIG. 6 shows a horizontal reactor arrangement.

FIG. 7 shows gas inlet ports for a horizontal reactor arrangement.

FIG. 1 is a schematic of the main elements of the reactor. The reactorcomprises an inner ceramic reactor core or vessel 1, a gas permeablebarrier 2, a gas inlet (for feed gas) 3, a gas outlet (for off-gas) 4and a product outlet port 5. In the preferred embodiment an FeNicatalyst (e.g. a Raney metal catalyst of the type sold by H. C. Starck,GmbH & Co. AG, Goslar, Germany under the trademark Amperkat

) is introduced into the reactor through catalyst inlet port 6 into thereaction region 7.

The reactor core 1 is preferably manufactured from Ceramite

(a castable high temperature ceramic material) and is surrounded by anouter shell 8 which is preferably manufactured from a high temperaturesteel.

The cavity 9 between the outer shell and the core is filled with amineral wool insulating material to insulate the steel casing 8 from theceramic core 1.

In operation the outer shell is pressurised to equal the pressure withinthe reactor core. Equal pressures on the inner and outer walls of theceramic core reduces the stress within the ceramic material. The outershell also provides connections for the carbonaceous gas inlet port,catalyst inlet port, product outlet port and gas outlet port.

A ceramic, gas-permeable barrier 2 is arranged at the top of the reactorand extends across the entire cross-section of the reactor core. Thebarrier is manufactured with a plurality of pores or apertures whichallow the gas to pass through the barrier and out of the reactor. In theproduction of CNF with a product size of between 1.5 mm and 8 mm and acatalyst size of 0.1 mm the pores are small enough to prevent thecatalyst and product from passing through the barrier.

The economy of the reactor is linked to a ratio (D) of carbon depositedto catalyst used and the average carbon deposition rate (H_(m)) becausethe purity in the final carbon product and the catalyst costs rise withD. The reactor size and degree of complexity rises with D/H_(m).

Typically, the reactor volume (Vr_(r)) in m³ for a production rate of Rtonnes/hour is given by:

V _(r) =D.R/(2k _(v) ·H _(m)·σ)

where:V_(r) reactor volume (m³)D carbon deposition degree (kg carbon per kg catalyst)R carbon production rate (tonnes/hour)k_(v) k_(v) is a correction factorH_(m) average carbon deposition rate (kg carbon per kg catalyst perhour)σ geometric density.

Setting the correction factor k_(v) at 1 gives the theoretical minimumreactor volume for a production rate R. This can be achieved in areactor which is run on a batch-wise basis until the reactor plugs orthe catalyst is completely deactivated, i.e. when there is no furthermethane conversion. In an industrial-scale reactor, the reactor shouldpreferably produce continuously and the carbon must be taken out of thereactor before the catalyst is deactivated, otherwise the reactor volumewill be unnecessarily large because H_(m) goes to zero. A productionrate of 20 tonnes/h of CNF in an industrial reactor (e.g. k_(v)=0.5)typically gives a reactor volume of 150 to 200 m³ when typical valuesfor catalysts are selected (e.g. D=200 kgC/kg catalyst and H_(m)=45kgC/kg catalyst per hour and the geometric density o=0.5).Realistically, the total reactor volume where k_(v)=0.5 for a productionrate of 20 tonnes/hour can thus be about 400 m². This gives a catalystusage of R/D=100 kg/hour when D=200 and leads to the case whereR·D/2H_(m)=44 tonnes of carbon in the reactor bed. In practice aproduction rate of 20 tonnes/hour would generally be split betweenseveral reactors.

In operation, carbonaceous gas (e.g. 90% mole methane and 10% molehydrogen) at a pressure of 10 bar is introduced into the gas inlet port3 of the reactor. A further one of the plurality of inlets 3 shown inFIG. 1 may be a carbon monoxide feed at a lower temperature than themethane feed. The gas flows vertically through the reactor and out ofthe gas outlet port 4.

An FeNi catalyst is introduced into the reactor through port 6 and intothe gas stream through a distribution nozzle 24 (as shown in FIG. 2)which distributes the catalyst evenly over the cross-section of thereaction region 7. A gas flow rate between the gas inlet and gas outletfor a given reactor size is selected so as to suspend the catalyst belowthe gas permeable barrier 2 in the reaction region 7. The pores orapertures within the barrier are sufficiently small to prevent thecatalyst and CNF product from travelling through the gas permeablebarrier but allow the gas to pass through the barrier.

The reaction taking place within a CNF producing reactor is thedecomposition of methane into carbon and hydrogen, i.e.

CH₄—>C+2H₂

The reaction is endothermic with hydrogen as a by-product and requiresthat the reaction zone be heated, typically to a temperature of at least650

C. The carbon product grows on the FeNi catalyst, and experiments show agrowth ratio of 1:200. The carbon growth will end when the grown carbonobstructs the supply of methane to the FeNi catalyst.

The carbon nano-fibres grow on the surface of the FeNi catalyst whichare suspended in the reaction region. In the reactor shown in FIGS. 1, 2and 3, the fibres grow until they are too heavy to be suspended by theflow of gas and then fall to the bottom of the reactor and out of thereactor and are removed through the particulate product outlet port 5.

The gas leaving the reactor through outlet 4 is partially recycled andfed back into the reactor through inlet 3. The presence of too muchhydrogen in the inlet gas reduces the carbon formation rate and hydrogenis therefore separated from the recycled outlet gas using a palladiummembrane (not shown).

During operation, the apertures within the barrier 2 through which thegas flows may become blocked with carbon particles produced in thereaction process. Intermittently applying a reverse flow of gas to thetop of the reactor means that the pores in the gas permeable barrier 2can be cleared.

FIG. 2 shows a cut-away of the core 1 showing the electrical heatingcoils 21 integrated into the ceramic reactor wall.

Before entering the reactor, the gas is first pre-heated by passing thegas through a heat exchanger (not shown) which exchanges heat from theoutlet gas so as to reduce the heating requirements of the electricalheating coils 21. The electrical heating coils 21 then raise the gastemperature to the operational temperature for CNF production.

As shown in FIG. 2, a cooling section 22 is provided between the reactorand a CNF product handling unit (not shown). The cooling section 22includes a cooling cavity 23 in which a coolant flows to cool theproduct as it passes through the section 22.

The cooled carbon enters the product handling unit (not shown) where awheel feeder fills a lock-chamber. The wheel feeder operates at zeropressure differential, and the lock-chamber therefore operates at thesame pressure as the reactor. Downstream of the lock-chamber, a furtherchamber, separated by a valve, is provided. The second chamber is usedto depressurise and flush the carbon before it leaves the processequipment.

FIG. 3 shows the three sections of a preferred embodiment of thereactor. The first or lower section 31 is arranged at the bottom of thereactor, has a conical shape and defines the product output port 5 whichis arranged vertically beneath the reaction region 7. Inlet gas issupplied into the reactor through a plurality of orifices 34 arrangedaround the periphery of the lower section 31.

The gas flows into the lower section 31 through gas inlet 3 and orifices34 and through the reduced cross-section middle section 32 where it isheated by the heating coils 21 (shown in FIG. 2).

The gas then flows into the third or upper section 33 which has aninverted conical shape and defines the reaction region 7 and acts as awind sieve. The upper limit of the third section 33 is defined by thegas permeable barrier 2 which extends across the cross-section of thethird section.

CNF is generated in the reaction region 7 and falls under gravitythrough the middle and lower sections 32, 31 and out of the reactorthrough product outlet port 5.

The arrangement of the conical lower section 31, the cylindrical middlesection 32 and the upper inverted conical section 33, makes it possibleto retain the carbon product and catalyst in the upper section 33 by thehigh gas flow rate in the cylindrical middle section 32 of the reactor.The reduced cross-sectional area of the middle section increases the gasvelocity which holds the carbon product in the upper section until theamount of carbon deposited on the catalyst particle has increased theweight of the catalyst particle to the extent that the upward flow ofgas flowing through the middle section 32 can no longer support theparticle. The middle section 32 in combination with the upper section 33thus acts as a wind screen allowing only particles having a certainweight through the middle section and to the lower section 31. When acatalyst particle with carbon deposits passes through the middle sectioninto the lower section the gas velocity in the lower section is lowerand the particle will fall to the product outlet port 5. Regulating thevelocity of gas in the middle section 32 can thus be used to regulatethe weight of the particles leaving the upper reaction region 7.

The reactor provides a continuous flow process for producing carbonnano-fibres. Catalyst can be introduced into the reactor using a batchfeed catalyst pre-treatment unit (not shown).

Controlling the flow of gas through the reactor can control the level atwhich the catalyst and product hover in the reactor and also the sizeand weight of products which are discharged.

The reactor can be used as both as an inverted fluidised bed reactor andalso an inverted fixed bed reactor by controlling the gas flow rate.

In an inverted fluidised bed mode of operation the reaction region isformed beneath the gas permeable barrier with an area (or wind sieve) ofno reaction between the reaction region and the gas permeable barrier.Increasing the gas flow rate will move the reaction region towards thegas permeable barrier until it is held against the gas permeablebarrier. An inverted fixed bed reaction region is thereby formed inwhich a product can grown and which can be discharged from the outletport 5 when the product grows to a size which can no longer be supportedby the gas flow.

The product outlet 5 (shown in FIG. 3) feeds into a product removal unit(not shown). The removal unit at the bottom of the reactor should beable to remove the carbon product from the reactor in a safe manner. Asthe reactor is pressurised, the removal unit should retain the pressurewithin the reactor during the removal process. In addition, theexplosive atmosphere surrounding the carbon should be vented off andpurged with nitrogen before the carbon leaves the unit.

FIG. 4 shows a serial arrangement of reactors. The reactors canadvantageously be arranged so that the outlet gas from a first reactor,optionally after hydrogen removal, can serve as the inlet gas for asubsequent reactor.

Reactors 41, 42, 43 each have gas outlets 44, 45, 46. Gas outlet 44feeds, via heat exchanger 47, the gas inlet 48 of the second reactor 42.Heat exchanger 47 acts to pre-heat the gas before entering thesubsequent reactor to ensure that each reactor receives gas at thecorrect temperature. Similarly gas outlet 45 of the second reactor 42flows, via heat exchanger 47, to gas inlet 49 of the third reactor 43.Gas outlet 46 of the third reactor 43 is fed to an off-gas handlingsystem (not shown) and returned to the first reactor 41 gas inlet 50.The hydrogen removal units are not shown.

Any number of reactors can be arranged in series provided that the gaspressure leaving a first reactor is sufficient to suspend the reactionregion in the subsequent reactor. Advantageously this arrangement can beused to produce a range of product sizes from each reactor productoutlet ports 51, 52, 53 in the series by controlling the reactionconditions within each of the separate reactors, i.e. the temperatureand pressure within each reactor in the series.

An alternative reactor for the production of particulate products suchas CNF is shown in FIG. 5.

FIG. 5 shows a preferred embodiment of a reactor having a cascadingarrangement. The reactor has an outer vessel 55 surrounding threereaction surfaces 56, 57, 58 onto which inlet gas is fed through inletconduits 59, 60, 61 respectively. The gas is dispersed onto the reactionsurfaces using nozzles 62 disposed on the reaction surfaces.

The gas is removed from the reactor though gas outlet 63 and particulateproduct is removed from the bottom of the reactor through product outletport 64.

In operation an FeNi catalyst is introduced into the reactor through acatalyst inlet port (not shown) and reacts with the inlet gas (such asmethane) on the horizontal reaction surfaces 56, 57, 58. As theparticulate product grows it covers the upper reaction surface and fallsonto the reaction surface below (the reaction surface below having alarger area than the reaction surface above, as shown in FIG. 5).Particulate product cascades over the edges of each of the reactionsurfaces and eventually over the edge of reaction surface 58 where isfalls out of the reactor through product outlet port 64.

The particulate product can therefore be harvested using gravity as theproduct reaches the edges of the reaction surfaces and falls out of thebottom of the reactor into a product collection area or zone.

A further alternative reactor for the production of particulate productssuch as CNF is shown in FIG. 6 in which the reaction bed can be fed withgas along the length of the reaction bed.

FIG. 6 shows a schematic of a horizontal reactor vessel 65 having a gasinlet port 66 and a gas outlet port 67.

The reactor also has a plurality of gas inlet ports 68, 69, 70, 71disposed along the length of the vessel through which inlet gas such asmethane is introduced into the reaction bed 72 shown in FIG. 7.

The reaction catalyst can be introduced into the reactor through the gasinlet port 66 or, alternatively, through a separate catalyst inlet portor nozzle (not shown) arranged in or proximate to the reaction bed 72.

FIG. 7 shows a cross-section of the reactor shown in FIG. 6. FIG. 7illustrates that the gas inlet ports may be arranged around thecircumference of the reactor shown by references 681-689 (FIG. 7) aswell as along the length of the reactor shown by references 68-71 (FIG.6).

Gas inlet ports around the periphery of the reactor support the reactionbed 72 and also supply feed gas for the reaction. The reactor cantherefore operate as a fixed bed or fluidised bed reactor by controllingthe flow rate of gas through the peripheral gas inlet ports shown inFIG. 7 and in particular the gas inlet ports arranged beneath thereaction bed 72.

In operation, heated methane gas is fed into the reactor through gasinlet 66. In addition, and as discussed above, methane gas is alsointroduced along the length of the reaction bed through holes 68, 69,70, 71 in the reactor walls and around the periphery of the reactor asshown in FIG. 7.

In this arrangement of reactor, compression of the reaction bed 72 slowscarbon formation. The reactor may therefore be provided with means toagitate the catalyst bed. Such agitation may be effected by the gas flowthrough the bed (as shown in FIG. 7) or the reactor may be provided withmoving or static mixers downstream of the start of the catalyst bed (notshown). The product is removed from the reactor by the flow of gasbetween the gas inlet port 66 and gas outlet port 67 and is preferablycollected by a filter or cyclone arranged in the outlet gas stream fromthe reactor.

Alternatively, some product and indeed some of the outlet gas may beremoved along the length of the reactor through ports (e.g. 681 to 689in FIG. 7) arranged to function as outlet rather than inlet ports.

Where the reactor is operated in a batchwise mode of operation, thecarbon generation process may be slowed down or halted towards the endof each batch by compression of the catalyst/carbon bed, either activelyor passively by allowing the catalyst/carbon bed to compress itselfagainst the end of the reaction zone in the reactor.

It will be appreciated that many of the features disclosed herein withreference to one arrangement of reactor can equally be applied to eachof the other arrangements of reactors. For example, the catalystsdiscussed with reference to the first reactor can equally be applied tothe reactor shown in FIGS. 5, 6 and 7.

It will also be appreciated that the reactors described herein, and withreference to the drawings, can be used for the production of polymers,especially polymers of ethylenically unsaturated hydrocarbons,particularly olefin polymers. The reactor could therefore be used as apolymerisation reactor for the production of plastics.

1. A reactor comprising a vessel having a plurality of gas inlet ports,a gas outlet port and a particulate product outlet port, wherein in usea reaction bed is formed in said vessel containing a bed ofcatalyst-containing particulate material and said gas inlet ports aredisposed so as to introduce gas into the reaction bed.
 2. A reactor asclaimed in claim 1 wherein the gas is introduced directly into thereaction bed.
 3. A reactor as claimed in claim 1 wherein the gas isintroduced through ports in the vessel wall proximate the reaction bed.4. A reactor as claimed in claim 1 wherein the reactor vessel isarranged in a horizontal orientation.
 5. A reactor as claimed in claim 1wherein the reactor vessel is arranged at an angle up to 45̂ from thehorizontal.
 6. A reactor as claimed in claim 1 comprising gas inletports arranged such that in use gas flow therefrom suspends the bed ofcatalyst-containing particulate material in said vessel and particulateproduct is discharged from the reactor vessel by falling from the bedand through one or more particulate product outlet ports.
 7. A reactoras claimed in claim 1 wherein product outlet ports may be arranged alongthe base of the vessel in the direction of travel of the bed such thatparticulate products can be harvested from the reactor by falling fromthe bed.
 8. A reactor as claimed in claim 1 wherein the vessel comprisesa downstream end and the gas outlet port and the particulate productoutlet port are a common outlet port at the downstream end of thevessel.
 9. A reactor as claimed in claim 1 comprising gas inlet and/orgas outlet ports above and/or along the reaction bed.
 10. A reactor asclaimed in claim 1 arranged so as to have an increasing cross-sectionalarea in the direction of gas flow.
 11. A reactor as claimed in claim 1wherein the vessel is cylindrical or conical in shape.
 12. A reactor asclaimed in claim 1 comprising gas inlet ports arranged tangentially tothe reactor vessel so as to agitate or spin the reaction bed.
 13. Areactor as claimed in claim 1 wherein the reactor is static.
 14. Areactor as claimed in claim 1 arranged to rotate so as to agitate thereaction bed.
 15. A reactor as claimed in claim 14 wherein the reactorvessel has an inside and an outside and wherein the inside of thereactor vessel is provided with stirring members connected to the insideof the reactor vessel such that the bed is agitated and stirred as thevessel rotates.